1. Field of the Invention
The present invention relates to lasers. More specifically, the present invention relates to high-efficiency lasers and methods for fabricating same.
2. Description of the Related Art
Highly efficient lasers are needed for applications ranging from active sensors to high-energy lasers for directed energy weapons. Due to the high efficiency thereof, solid-state lasers have proven to be very useful for these applications. Unfortunately, when scaled to high average power levels, waste heat generated by these lasers typically cause thermal gradients in the gain media thereof. As the thermal gradients diminish the quality of the output beam, a heat management system is typically required in conjunction with systems for correcting the beam distortion caused by the waste heat. The waste heat is due to a non-ideal conversion of pump light to laser light. This defect is referred to as a ‘quantum defect’, the ratio of the photon frequency or energy of the laser divided by the photon frequency or energy of the pump.
Conventional approaches to the problem of waste heat in solid-state lasers include efforts to increase the efficiency thereof. Neodymium YAG lasers for example have shown efficiencies of approximately 70%, close to the quantum defect thereof. However, the sensible heat for these lasers remains substantial.
Ytterbium YAG lasers have also shown encouraging efficiency levels at or near 80%. However, scaling of these lasers to kilowatt levels requires a sophisticated thermal management system notwithstanding the efficiency thereof.
Radiation balanced or non-exothermic laser systems such as that disclosed in U.S. Pat. No. 6,370,172, issued Apr. 9, 2002 to S. R. Bowman, the teachings of which are hereby incorporated herein by reference, have been considered. This reference discloses a resonantly pumped laser in which the upper laser state manifold is the level that is being pumped into. That is, in a two level system, the pump manifolds have sublevels known as ‘Stark energy levels’. Pumping into the Stark energy levels yields lasing action at different Stark energy levels. This approach is referred to a ‘resonant pumping’ and Yb3+:YAG and Er3+:YAG lasers—for example—are referred to as ‘resonant pumped lasers’.
With the radiation balanced approach, an Ytterbium (Yb3+) based crystal laser gain medium is chosen so that the pump frequency is lower than the mean florescence frequency so that a balance is achieved whereby the mean florescence frequency is equal to the pump laser frequency plus the laser frequency. Unfortunately, this approach is challenging and problematic in that the florescence rates must be fast enough in order to meet the second requirement: namely, the spontaneous emission rate times the mean fluorescence frequency plus the stimulated emission (laser emission) rate times the laser frequency must be equal to the pumping rate times the pump frequency. Since fluorescence emission is the means for cooling, the gain geometry must be such that the fluorescence emission is effectively dispatched outside the lasing medium in order to prevent re-absorption into the gain medium which would diminish the cooling effect. This typically puts severe restrictions on the laser gain geometry such that at least one optically thin dimension is required.
Consequently, this approach is heavily dependent on florescence cooling to achieve a low heat laser design. Florescence cooling approaches are limited in that generally, three requirements that must be met: 1) the mean fluorescence frequency must be higher than the pump frequency; 2) the fluorescence rates must be sufficiently high as mentioned above; and 3) the florescence re-absorption must be minimized for the waste heat to escape.
Hence, a need remains in the art for a highly efficient laser with a substantially reduced need for thermal management.